Data centers contain information and communications technology (ICT) equipment, such as server and network equipment as well as computers. Depending on the design of such equipment, it generates a comparatively large quantity of sensible heat that must be continually removed so as to maintain the data center climate within a design range of temperature and humidity.
Since mechanical climate control systems relying solely on refrigerant or liquid-based cooling are costly and have a negative impact on the environment, there has been an impetus toward the use of heat exchange between cooler ambient air and the heated recirculating air within the data center. Leveraging the heat exchanger paradigm, data centers are able to limit the use of mechanical cooling and rely wholly or in part on temperature differential to provide adequate heat rejection. This concept is known as “free-cooling”. Such designs have employed an air-to-air heat exchanger of the rotary type, or heat wheel, comprising a rotating disc-shaped perforated metallic plate, which extends through a partition separating the primary recirculating air stream from the secondary outside air stream.
The use of a rotary heat wheel, however, involves several disadvantages. Inherent in the design is air leakage between the primary and secondary air streams through the breach in the partition needed to accommodate the rotating heat wheel. Since brushes and air sealing gaskets cannot completely stop leakage, energy transfer efficiency is diminished. Additionally, information and communications technology (ICT) equipment is often very sensitive to fluctuations in humidity and the presence of airborne contaminates and/or particulate. Since, leakage is an inherent characteristic of the heat wheel, this method results in unfiltered/unconditioned ambient air infiltrating into the critical space where ICT equipment is housed, which is highly undesirable.
The heat wheel must continuously rotate in order to realize any heat transfer potential. Significant electrical energy is consumed in powering the electric motor that rotates the heat wheel. Moreover, the heat wheel consists of moving parts which must be regularly inspected and maintained by trained professionals. The passage of both air streams through the perforations of the heat wheel causes a substantial pressure drop, which must be compensated for by providing more fan power. Heat wheels also require life-cycle replacement, typically after ten years. Therefore, inherent leakage characteristics, utility costs, annual cost of ownership, and relatively short lifespan of the heat wheel have rendered this approach impractical for many data centers.